(1) Field of the Invention
This invention relates to apparatus and methods for decoding a bit stream using synchronization information contained in two synchronization marks embedded in the bit stream and more particularly to the capability of using the synchronization information in either of the synchronization marks.
(2) Description of the Related Art
When data is retrieved from a storage media the beginning and end of the data stream must be correctly determined. This is accomplished using synchronization data contained in two synchronization marks. If the separation between the first and second synchronization marks is large there is a possibility that large amounts of data can be lost if there is a failure in decoding the first synchronization mark.
U.S. Pat. No. 5,796,690 to Kanno describes a disc controller with improved data sync and re-sync mark detection. In response to sync and re-sync mark detection failure status signals, a re-sync mark detection window expands a timing window opening to improve the possibility of detection of the next re-sync mark.
U.S. Pat. No. 5,844,920 to Zook et al. describes a method for thermal asperity compensation using multiple sync marks for retroactive and split segment data synchronization in a magnetic disk storage system. In a magnetic disk storage system, byte synchronization to sector data is achieved even when noise in the read channel, due for instance to a thermal asperity (TA), corrupts the primary preamble and/or sync mark fields or causes a loss of frequency or phase lock. The data sector format is modified to comprise at least one secondary sync mark in addition to the conventional primary sync mark recorded at the beginning of the data field. In this manner, when the primary sync mark becomes undetectable due to errors, or when byte synchronization is lost, the storage system can still synchronize to the data sector using the secondary sync mark.
U.S. Pat. No. 6,009,549 to Bliss et al. describes a disk storage system employing error detection and correction of channel coded data, interpolated timing recovery, and retroactive and split-segment symbol synchronization. The disk storage system allows retroactive and split-segment symbol synchronization using multiple sync marks.
During the process of data retrieval from a storage media, such as magnetic disk drive, it is necessary to correctly determine the beginning and the end of the data stream. This is typically done by using synchronization information encoded in the special synchronization marks which are embedded in the data stream. FIG. 1 shows a schematic representation of the beginning of a data sector on a disk drive. The sector begins with a preamble pattern 100 which is required by the timing and gain recovery means of the data retrieval circuitry. Next the first synchronization mark 101 indicates the beginning of the encoded data bytes 102 and 104. The data bytes also contain the second synchronization mark 103, which is used to prevent failure of the data synchronization mechanism if the first synchronization mark is not detected due to problems such as media defects or high noise conditions. In the event the first synchronization mark 101 is not detected, the portion of the data 102 prior to the first synchronization mark 101 is lost since no synchronization information is available. However, the second portion 104 of the data is successfully retrieved since the second synchronization mark 103 provides synchronization information for the second portion 104 of the data.
FIG. 2 shows a schematic drawing of conventional data retrieval circuitry. The retrieved signal samples first enter the front end portion 200 where a variety of operations, such as filtering and timing recovery, are performed. The modified samples are then sent to a detector circuit 201 where the bit stream taken from the media is reconstructed using the samples. This bit stream is then sent to the rest of the data retrieval circuitry 203 and also fed to the synchronization detector 202. The synchronization information provided by the synchronization detector 202 decoding the synchronization mark is used to decode the bit stream. The disadvantage of the conventional data retrieval circuitry, shown in FIG. 2, is that failure in detecting the first synchronization mark, 101 in FIG. 1, results in the loss of the data, 102 in FIG. 1, prior to the second synchronization mark, 103 in FIG. 1.
It is a principle objective of this invention to provide a method of decoding a bit stream having two synchronization marks which avoids loss of data if the detection operation of one of the synchronization marks fails.
It is another principle objective of this invention to provide an apparatus for decoding a bit stream having two synchronization marks which avoids loss of data if the detection operation of one of the synchronization marks fails.
These objectives are achieved by feeding the bit stream into a front end circuit. The output of the front end circuit is fed to a first memory so that the entire bit stream is stored in the first memory. The output of the first memory is fed to a first stage detector. The first stage of detection does not require synchronization information. The output of the first stage detector is fed to a second memory and simultaneously to a synchronization detector. The synchronization detector decodes the synchronization information from either the first or second synchronization mark. Later stages of detection are then carried out using the synchronization information from the synchronization detector and the data stored in the first memory and second memory.
This method prevents the loss of data between the first and second synchronization marks in the event the first synchronization mark data is lost, since all the data from the bit stream are stored in first memory and can be accessed later using the synchronization information from the second synchronization mark.